1. Field of the Invention
This invention relates to the termination of electrical traces. The invention is more particularly related to the termination of traces on a card connected to a backplane. The invention is still further related to programmable resistive terminations that either terminate or do not affect traces on a card. The invention is yet further related to a line card having traces with programmable terminations controlled by a command message.
2. Discussion of the Background
Modern electronic intensive systems come in many configurations. Typically, large electronic intensive systems include a chassis having a backplane and line cards. The backplane carries electrical lines to the line cards, and normally includes a bus shared between the line cards. The line cards have connectors that physically attach the line card to the chassis and electrically connect the line card and its associated electronic devices to the backplane and bus.
One modern electronic system having such a configuration is an access device utilized by a local telephone company to access voice traffic from a high capacity network. FIG. 1 illustrates an example of an access device (terminal unit) 150 installed at a central office 100 of a local telephone company.
In FIG. 1, a narrowband switch 110 resident in the Central Office 100 is connected via a fiber link to a narrowband network 120 carrying time-domain multiplexed (TDM) traffic. The narrowband network 120 includes addition links to long distance lines 130, for example. The voice switch 110 routes narrowband (voice and data, for example) traffic to and from a terminal unit 150 via connecting cable 140. The terminal unit 150 multiplexes signals between the voice switch 110 and customers 1 . . . m 160.
FIG. 2 illustrates the terminal unit 150 in greater detail. A control shelf 200 sends and accepts signals (narrowband traffic in this example) to/from the connecting cable 140. The control shelf 200 multiplexes the signals between the connecting cable and plural bank controller units (BCUs) 2201 . . . 220n. Each BCU is located on a respective shelf of channel bank shelves 2101 . . . 210n. Each of shelves 2101 . . . 210n support one or more rows (row 230, for example) having slots s1 . . . sp of row 230 for installing line cards.
Each BCU multiplexes signals between its respective rows of line cards and the control shelf 200. Thus high density traffic received by the control shelf 200 is multiplexed to plural BCUs, and the BCUs multiplex traffic to individual line cards installed in line card rows maintained in a respective shelf of the BCU. The individual line cards communicate traffic between the terminal unit 150 and customers 1 . . . m. Traffic from customers 1 . . . m to the narrowband network 120 is handled in reverse order.
As with electronic devices of similar physical configuration, each of shelves 2201 . . . 220n include a backplane. The backplane provides signal lines to communicate data and control information between the BCU and individual cards, or between any two or more cards in each row, depending on the electrical configuration of the system, and typically includes at least one bus.
A backplane utilized by the terminal unit 150 is illustrated in FIG. 3. The BCU 220 is connected to a bus 300 that includes x individual lines for addressing and data. The bus 300 is connected to plural line cards (lci . . . lcp), and a bus termination 310. Referring to FIG. 4, the bus termination 310 provides a resistive termination to ground (R1 . . . Rx) for each of the individual lines 1 . . . x of the bus 300.
The bus terminations are vitally important in the operation of a high speed bus. The terminations sink signals, transmitted on the bus to ground, minimizing reflection of signals that reach the end of the bus. The resistence selected for the terminations is also extremely important because it affects the amount of current needed to drive transmitted signals, and can either inhibit or accelerate rise times of the signal carried on the bus.
The present inventor has realized that in certain situations it is not feasible to terminate backplane traces on the backplane and that the terminations may be accomplished on one of the line cards instead. This is the case, for example, where there is already a large installed base of systems having previously unused backplane traces which are unterminated, and with the advance of technology it is desirable to provide new line cards which utilize the previously unused traces. In the system of FIGS. 1–4, for example, the point-to-point subscriber bus was used exclusively for narrowband telephony traffic. Whereas these traces were terminated on the backplane a previously mentioned, the backplane also included a number of extra traces which were unused and unterminated. Technological advances have made it possible to carry much more information at much higher speeds (specifically ADSL “broadband” traffic) by taking advantage of the previously unused traces, through the creation of new bank control units (called ABCUs) and new line cards (called ADLUs) to be retrofitted into the installed base of systems. But in order to do so, the extra traces somehow need to be terminated. Otherwise the reflections and other noise that will appear on such traces will seriously impact the reliability of the new broadband traffic capacity.
One possibility, of course, would be to retrofit the backplanes in each system with new termination resistors on the extra traces. Such a solution would be commercially undesirable because of the enormous expense of sending numerous skilled technicians out to thousands of installations to perform the retrofit.
Another possibility would be to terminate the extra traces on the new line cards themselves. However, a single row 230 in a channel bank shelf 210, can contain any number of line cards (up to 20 in this example), and the number and physical placement along the backplane of the line cards will vary from system to system. If all line cards that are newly installed in a system are resistively grounding the backplane traces, then the loading on the trace would be unpredictable (because the number of new line cards is unpredictable), and typically much too heavy.
It might be possible to manufacture some line cards that do have resistive terminations and some that do not, but then certain economies of scale in production would be lost. In addition, because the terminations should be a close to the end of the backplane traces a possible (farthest from the ABCU), the effectiveness of the terminations, and therefore the reliability of broadband communications within the system, would depend on the uncertain dependability of each system operator installing the correct type of line card in the correct slot on the backplane.
Yet another possibility might be to include termination resistors on all of the new line cards, and provide a switch for a technician to activate or deactivate the terminations on a card-by-card basis. This solution avoids degradation of production economies, because all line cards re identical, but again depends for its effectiveness on the reliability of each system operator to activate the termination resistors on the card farthest from the ABCU, and only on that card.
It can be seen that the promise of high speed broadband telephony traffic enhancing a large installed base of conventional telephony systems, might be realizable only if the problem of resistive terminations of previously unterminated spare backplane traces can be solved properly. There is therefore a strong need for a technique for terminating such backplane traces which is commercially feasible, which does not risk heavy or unpredictable loading of the traces, which does not degrade production economies, and which does not depend for its reliability on system operators' manual tasks when installing new line cards into an existing system.